The present invention relates to a nitride semiconductor device having a function such as light emission, which includes a nitride based semiconductor crystal layer formed by crystal growth and an electrode layer formed on the crystal layer, and a fabrication method thereof, and particularly to a nitride semiconductor device capable of realizing efficient current injection and a fabrication method thereof.
Nitride based III-V compound semiconductors such as GaN, AlGaN, and GaInN, each of which has a forbidden band width ranging from 1.8 eV to 6.2 eV, become a focus of attention in theoretically realizing light emitting devices allowing emission of light in a wide range from red light to ultraviolet right.
In fabrication of light emitting diodes (LEDs) and semiconductor lasers by using nitride based III-V compound semiconductors, it is required to form a stacked structure of layers made from GaN, AlGaN, GaInN and the like, wherein a light emitting layer (active layer) is held between an n-type cladding layer and a p-type cladding layer. As one example, there is known a light emitting diode or a semiconductor laser including a light emitting layer having a GaInN/GaN quantum-well structure or a GaInN/AlGaN quantum-well structure.
A vapor-phase growth technique for a nitride semiconductor such as a gallium nitride based compound semiconductor has an inconvenience that there is no substrate allowed to be lattice matched with a nitride semiconductor or a substrate having a low density of dislocations. To cope with such an inconvenience, there has been known a technique of depositing a low temperature buffer layer made from AlN or AlxGa1-xN (0xe2x89xa6x less than 1) at a low temperature of 900xc2x0 C. or less on a surface of a substrate made from sapphire or the like, and then growing a gallium nitride based compound semiconductor thereon, thereby reducing dislocations due to lattice mismatching between the substrate and the compound semiconductor. Such a technique has been disclosed, for example, in Japanese Patent Laid-open No. Sho 63-188938 and Japanese Patent Publication No. Hei 8-8217. By using such a technique, it is possible to improve the crystallinity and morphology of a gallium nitride based compound semiconductor.
Another technique of obtaining high quality crystal at a low density of dislocations has been disclosed, for example, in Japanese Patent Laid-open Nos. Hei 10-312971 and Hei 11-251253. This method involves depositing a first gallium nitride based compound semiconductor layer, forming a protective film made from a material capable of inhibiting growth of a gallium nitride based compound semiconductor, such as silicon oxide or silicon nitride, and growing a second gallium nitride based compound semiconductor in an in-plane direction (lateral direction) from regions, not covered with the protective film, of the first gallium nitride based compound nitride layer, thereby preventing propagation of threading dislocations extending in the direction perpendicular to the interface of the substrate. A further technique of reducing a density of threading dislocations has been disclosed, for example, in a document (MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999), or Journal of Crystal Growth 189/190 (1998) 83-86). This method involves growing a first gallium nitride based compound semiconductor, selectively removing the thus formed semiconductor film by using a reactive ion etching (hereinafter, referred to as xe2x80x9cRIExe2x80x9d) system, and selectively growing a second gallium nitride based compound semiconductor from the remaining crystal in the growth apparatus, thereby reducing the density of threading dislocations. By using these techniques, it is possible to obtain a crystal film having a density of dislocations, which is reduced to about 106/cmxe2x88x922, and hence to realize a high life semiconductor laser using the crystal film.
The selective growth is useful not only for reducing threading dislocations as described above but also for producing a semiconductor device having a three-dimensional structure. For example, a semiconductor device having a three-dimensional structure can be obtained by forming an anti-growth film on a gallium nitride based compound semiconductor film or a substrate, and selectively growing crystal from an opening portion formed in the anti-growth film, or by selectively removing a gallium nitride based compound semiconductor film or a substrate, and selectively growing from the remaining crystal. Such a semiconductor device has a three-dimensional structure having a facet composed of side planes and a top (upper surface) at which the side planes cross each other, and is advantageous in reducing a damage in the device isolation step, easily forming a current constriction structure of a laser, or improving the crystallinity by positively using characteristics of crystal planes forming the facet.
FIG. 30 is a sectional view showing one example of a nitride based light emitting device grown to a three-dimensional shape by selective growth, wherein the light emitting device is configured as a GaN based light emitting diode. An n-type GaN layer 331 is formed as an underlying growth layer on a sapphire substrate 330. A silicon oxide film 332 having an opening portion 333 is formed on the n-type GaN layer 331 so as to cover the n-type GaN layer 331. A hexagonal pyramid shaped GaN layer 334 is formed by selective growth from the opening portion 333 opened in the silicon oxide film 332.
If the principal plane of the sapphire substrate 330 is the C-plane, the GaN layer 334 becomes a pyramid shaped growth layer covered with the S-planes ({1,xe2x88x921,0,1} planes). The GaN layer 334 is doped with silicon. The tilted S-plane portion of the GaN layer 334 functions as a cladding portion. An InGaN layer 335 is formed as an active layer so as to cover the tilted S-planes of the GaN layer 334, and an AlGaN layer 336 and a GaN layer 337 doped with magnesium are formed on the outside of the InGaN layer 335.
A p-electrode 338 and an n-electrode 339 are formed on such a light emitting diode. The p-electrode 338 is formed on the GaN layer 337 doped with magnesium by vapor-depositing a metal material such as Ni/Pt/Au or Ni(Pd)/Pt/Au. The n-electrode 339 is formed in an opening portion opened in the silicon oxide film 332 by vapor-depositing a metal material such as Ti/Al/Pt/Au.
FIG. 31 is a sectional view showing one example of a related art nitride based light emitting device grown into a three-dimensional shape by selective growth. Like the nitride semiconductor light emitting device shown in FIG. 30, an n-type GaN layer 351 is formed as an underlying growth layer on a sapphire substrate 350. A silicon oxide film 352 having an opening portion 353 is formed on the n-type GaN layer 351 so as to cover the n-type GaN layer 351. A hexagonal column shaped GaN layer 354 having a rectangular shape in cross-section is formed by selective growth from the opening portion 353 opened in the silicon oxide film 352.
The GaN layer is a region doped with silicon, and is grown to a growth layer having side planes composed of the {1,xe2x88x921,0,0} planes by adjusting a growth condition for selective growth. An InGaN layer 355 is formed as an active layer so as to cover the GaN layer 354. A p-type AlGaN layer 356 and a p-type GaN layer 357 doped with magnesium are formed on the outer side of the InGaN layer 355.
A p-electrode 358 and an n-electrode 359 are formed on such a light emitting diode. The p-electrode 358 is formed on the GaN layer 357 doped with magnesium by vapor-depositing a metal material such as Ni/Pt/Au or Ni(Pd)/Pt/Au. The n-electrode 359 is formed in an opening portion opened in the silicon oxide film 352 by vapor-depositing a metal material such as Ti/Al/Pt/Au.
In the case of using such selective growth, however, there may occur an inconvenience that since the top or the upper surface is surrounded by the facet composed of the side planes low in growth rate, the supply of a source gas becomes too much on the top or the upper surface, tending to degrade the crystallinity of a portion on the top or the upper surface. Further, in the case where the area of the top or the upper surface is smaller than that of a substrate, it is difficult to control the film thickness and the composition of mixed crystal at the top or the upper surface. Accordingly, even in the case where a semiconductor light emitting device having a three-dimensional structure is formed by selective growth, there arise problems in degrading the crystallinity of the top or the upper surface, reducing the efficiency due to a nonradiative recombination, and causing leakage of a current because of irregular formation of PN-junction. Further, depending on the resistivity and thickness of a conductive layer being in contact with an electrode, a current is spread in the conductive layer, so that the current tends to be injected in the top or the upper surface, thereby degrading the device characteristics.
In the case of using selective growth, like the above-described top or the upper surface, a ridge portion as a crossing line portion between adjacent side surfaces and a region extending along the ridge portion, or a bottom side portion as a crossing line portion between a side surface and a bottom surface and a region extending along the bottom side portion are each poor in crystallinity. As a result, even on the ridge portion and the region extending along the ridge portion or the bottom side portion and the region extending along the bottom side portion, there may arise problems in reducing the efficiency due to nonradiative recombination, causing leakage of a current due to irregular formation of PN-junction, and the like.
Accordingly, a need exists to provide an improved nitride semiconductor device, such as to provide a nitride semiconductor device capable of obtaining excellent characteristics even if the device structure is grown to a three-dimensional shape by selective growth, and to provide a method of fabricating the nitride semiconductor device.
According to an embodiment of the present invention, a nitride semiconductor device is provided that includes a crystal layer grown in a three-dimensional shape having a side surface portion and an upper layer portion, wherein an electrode layer is formed on said upper layer portion via a high resistance region.
According to the nitride semiconductor device of an embodiment the present invention, a current for operating the nitride semiconductor device is injected from an electrode layer, and in this case, since the high resistance region is provided on the upper layer portion, the current flows so as to bypass the high resistance region of the upper layer portion, to form a current path extending mainly or substantially along the side surface portion while avoiding the upper layer portion. By using such a current path extending mainly along the side surface portion, it is possible to suppress the flow of the current in the upper layer portion poor in crystallinity.
According to an embodiment of the present invention, a nitride semiconductor device is provided that includes a crystal layer grown on a nitride semiconductor layer or a nitride semiconductor substrate, wherein said crystal layer includes a first crystal portion having a good crystal state and a second crystal portion having a crystal state poorer than that of said first crystal portion, and an electrode layer is formed on said second crystal portion via a high resistance region.
According to this nitride semiconductor device, since the electrode layer is formed on the second crystal portion via the high resistance region, a current path avoiding the second crystal portion is formed, to form a current path extending mainly via the first crystal portion good in crystallinity while bypassing the second crystal portion poor in crystallinity by the presence of the high resistance region. Accordingly, it is possible to make use of a portion good in crystallinity for an active device, and hence to optimize the device characteristics.
According to an embodiment of the present invention, a method of fabricating a nitride semiconductor device is provided that includes, the steps of forming a crystal layer on a nitride semiconductor layer or a nitride semiconductor substrate by selective growth, continuously forming a high resistance region by changing a crystal growth condition after formation of an upper layer portion of the crystal layer, and forming an electrode layer after formation of the high resistance region.
In the method of fabricating a nitride semiconductor device, said step of forming a crystal layer by selective growth includes the step of forming an anti-growth film on the nitride semiconductor layer or the nitride semiconductor substrate, and growing crystal from an opening portion opened in the anti-growth film, or includes the step of selectively removing part of the nitride semiconductor layer or the nitride semiconductor substrate, and growing crystal from the remaining portion of the nitride semiconductor layer or the nitride semiconductor substrate.
According to the method of fabricating a nitride semiconductor device of an embodiment the present invention, the crystal layer is formed by selective growth. In this case, the crystal layer is grown into a three-dimension al shape having an upper layer portion and a side surface portion by making use of selective growth. The resistance of the high resistance region formed continuously with this crystal growth becomes high by changing the crystal growth condition. Such a high resistance region functions to allow a current path from the electrode layer to bypass a portion poor in crystallinity. Since the high resistance region can be disposed in proximity to the upper layer portion of the crystal layer because it is formed continuously with the crystal growth of the crystal layer, it is possible to suppress the flow of a current in the upper layer portion poor is crystallinity.
According to an embodiment of the present invention, a nitride semiconductor device is provided that includes a crystal layer grown into a three-dimensional shape having a ridge portion, wherein an electrode layer is formed on both said ridge portion and a region extending along said ridge portion via a high resistance region. According to an embodiment of the present invention, there is also provided a nitride semiconductor device including a crystal layer grown into a three-dimensional shape, wherein an electrode layer is formed on both a bottom portion of said crystal layer and a region extending on said bottom portion via a high resistance region.
The high resistance region is formed by providing an undoped portion or an ion implanted portion formed by ion implantation, or selectively irradiating a nitride semiconductor layer doped with a p-type impurity with electron beams.
According to the nitride semiconductor device of an embodiment of the present invention, a current for operating the nitride semiconductor device is injected from an electrode layer, and in this case, since a high resistance region is formed on a ridge portion and a region extending along the ridge portion or a bottom side portion and a region extending along the bottom side portion, a current flows so as to bypass the high resistance region, to form a current path extending mainly along a side surface portion, concretely, along a flat surface portion composed of the side surface portion. By using such a current path, it is possible to suppress the flow of a current in the ridge portion poor in crystallinity and the region extending along the ridge portion or the bottom side portion and the region extending along the bottom side portion.
According to an embodiment of the present invention, a nitride semiconductor device is provided that includes a crystal layer grown into a three-dimensional shape, wherein an electrode layer is formed on a flat surface portion, other than a ridge portion and a region extending along said ridge portion, of said crystal layer. According to an embodiment of the present invention, there is also provided a nitride semiconductor device including a crystal layer grown into a three-dimensional shape, wherein an electrode layer is formed on a flat surface portion, other than a bottom side portion and a region extending along said bottom side portion, of said crystal layer.
According to the nitride semiconductor device of an embodiment of the present invention, a current for operating the nitride semiconductor device is injected from an electrode layer, and in this case, since the electrode layer is not formed on a ridge portion and a region extending along the ridge portion or a bottom side portion and a region extending along the bottom side portion, a current path mainly extending along a side surface portion on which the electrode layer is formed, concretely, along a flat surface portion composed of the side surface portion, can be formed. By using such a current path, it is possible to suppress the flow of a current in the ridge portion poor in crystallinity and the region extending along the ridge portion or the bottom side portion and the region extending along the bottom side portion.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.